Automatic demodulator sensitivity control



Jan. 14, 1936. R URTEL TAL 2,027,939

AUTOMATIC DEMODULATOR SENSITIVITY CONTROL Filed Oct. 2l, 1935 ATTORN EY UNiTED sr'ras PATEN OFFICE AUTOMATIC DEMODULATOR SENSITIVITY CONTROL Rudolf Urtel and Karl Steimel, Berlin, Germany,

assignors to Telefunken Gesellschaft fr Drahtlose 'll'.elegraphie m. b. H., Berlin, Germany, a corporation of Germany Application October 21, 1933, Serial No. 694,666 In Germany December 22, 1932 6 Claims.

The present invention relates to demodulation circuit arrangements comprising means to insure automatic dependence of sensitiveness upon signal strength.

5 Automatic compensation of volume variations due to iluctuations in the incoming signal strength, say due to fading, may be effected by that, by the aid of suitable devices, the amplication factor or gain of the radio frequency ampliiier below the demodulator is regulated in dependence upon the carrier-wave amplitude. Arrangements, however, have also been disclosed in the earlier art in which the sensitivity of the demodulator itself is regulated as a function of the l carrier amplitude. Arrangements of this sort are predicated upon the use of special plate currentgrid voltage or plate current-plate-potential characteristics according to the rectier method that is in use. These characteristics result in a certain Klirr (distortion) factor, and as a general rule they permit of a sumciently accurate compensation only inside relatively small uctuations.

Now, according to the present invention, a novel arrangement is suggested which is designed to obviate these drawbacks, as follows:

In a three-grid tube whose first grid, figured from the cathode, serves and acts as the control grid, whose second grid serves as a screen-grid,

and in which the third grid mounted between the said screen grid and the anode, or plate, is maintained at Zero potential or at a negative potential in reference to the cathode, the slope (mutual conductance) being the ratio: Plate current variation to control-grid potential-variation is changeable by the adjustment of the voltage of the auxiliary grid.

Certain secondary actions of a troublesome nature such as decreasing internal resistance with increasing negative auxiliary-grid voltage can be obviated by the introduction of a second screen grid between auxiliary grid and anode. However, so far as the principle here described is concerned, these phenomena are immaterial.

In the drawingz- Figures 1, 2 and 3 show different embodiments of the invention.

Now, if a tube of this kind is operated in a grid rectification circuit scheme (audion), as shown in Fig. 1, an audio component will arise at the control grid G1 which is amplied by the electrode system in the usual way. A change in sensitivity is effected by that the D. C. component arising across the grid resistance Rg of the control grid is wholly, or partly, impressed upon the auxil- (Cl. Z50- 27) iary grid G3 which varies the slope of the control grid. As the carrier-wave .amplitude grows, there occurs a rise both of the audio amplitude at the control grid as Well as the D. C. Voltage component. The consequence of this is a decrease or loss in slope of the control grid, so that, if dimensions have been chosen properly, the audio output voltage can be kept practically constant.

The identical principle is applicable, with but slight changes, if rectication is effected in a separate rectier, or rectifier path (diode, crystal detector, etc.) as shown in Fig. 2. The rectifier arrangement D again furnishes an audio component as well as a D. C. voltage component, the former one being impressed upon the control grid G1 and the latter one upon the auxiliary grid G3 regulating the slope of the control grid.

When using the basic idea of this invention in connection with a plate rectification circuit arrangement (Fig. 3), the operation would be thus: The control grid G1 is given a negative biasing voltage E'g so that the working point comes to lie in the lower bend of the characteristic curve and that plate rectification is insured. In the 25 plate circuit is inserted a resistance R immediately at the cathode; the fall of potential occasioned across the said resistance serves as a biasing voltage for the auxiliary grid changing the slope of the control grid. As the carrier-wave 30 amplitude grows, there also occurs a rise in the plate D. C., the voltage drop across the cathode resistance increases, and the slope of the control grid is diminished by the influence of the auxiliary grid potential shifted into the negative 35 region, so that, if dimensions have been properly chosen, stabilization of the audio output amplitude is possible.

What is claimed is:-

1. In combination with a source of signals to 40 be demodulated and a network for utilizing the audio component'of the demodulated signals, an electron discharge tube provided with a cathode,

a plate, a control grid adjacent the cathode, an

auxiliary grid adjacent the plate, and a positive 45 screen grid between the two grids, a second network, including a signal input circuit, coupled between said source and the cathode and control grid of said tube, means in said second network for developing from said signals, between said cathode and control grid, audio and direct current voltage components, and a direct current connection between the auxiliary grid and a direct current voltage point on said last means which is negative with respect to the cathode of 55 the tube 'whereby the said direct current voltage component is applied to said auxiliary grid as a gain regulation voltage for the tube, and said audio component being repeated into the plate circuit of the tube.

2. In a system as deiined in claim l, said direct current connection including a resistor.

3. In a system as defined in claim 1, said direct current connection including a resistor connected between the control and auxiliary grids, and a second series resistor connected between the cathode and control grid, said two resistors being shunted by a condenser.'

4. In a system as dened in claim 1, said second network means comprising a-rectiiier, and said control grid and cathode being coupled to the rectifier.

5. In combination with a signal demodulation diode, a source of signals coupled to the diode, a tube having a cathode, a plate, and three grids between them, the diode being coupled between one of the grids and the cathode to impress the audio component of demodulated signals therebetween, a direct current connection between the diode anode and a second of the grids, the diode cathode and said tube cathode being at the same direct current potential, and a third of the grids being disposed between the iirst and second grids and being at a positive potential with respect thereto.

6. In combination with an input network of electrical waves, an electron discharge tube of the type including a cathode, a plate, a signal control grid adjacent the cathode, a gain control electrode adjacent the plate, and a positive screen electrode between the control grid and gain control electrode, a signal utilization network coupled to the plate and cathode, said input network being coupled between the cathode and control grid, means, including a resistor having one 15 side thereof connected to said cathode, for deriving from said waves a direct current voltage whose value varies with the wave amplitude, and a direct current connection between the gain control electrode and the other side of said re- 20 

